Conventional seismic technology uses surface sources of seismic energy and surface receivers to detect reflections from underground geologic formations such as layers and faults. The collected seismic data can be used to locate the underground formations and guide drilling operations to sources of hydrocarbons.
In Vertical Seismic Profiling ("VSP"), drilling operations are interrupted to place a series of seismic sensors at discrete depths in a borehole being drilled. A surface source releases energy that is reflected off underground geologic formations. The seismic sensors in the borehole sense the reflected energy and provide signals representing the reflections to the surface for analysis. The seismic data is used in analysis of the geology of the earth near the borehole.
In a subsequent development, known as "drill bit seismics", seismic sensors were positioned at the surface near the borehole to sense seismic energy imparted to the earth by the drill bit during drilling. The sensed energy was used in the traditional seismic way to detect reflections from underground geologic formations. Further, this technique was used to detect "shadows", or reduced seismic energy magnitude, caused by underground formations, such as gas reservoirs, between the drill bit and the surface sensors.
Analysis of drill bit seismic data entails determining the amount of time between the generation of the seismic energy by the drill bit and the detection of the seismic energy. This can be accomplished by using a "pilot sensor" near the top of the drillstring to which the drill bit is attached. The pilot sensor detects the vibrations transmitted through the drillstring by the drill bit. Those vibrations, corrected and filtered to account for the delay and distortion caused by the drillstring, can be correlated with the seismic energy received at the surface sensors to determine the distance traveled by the seismic energy received at the surface sensors. With this information, the location of the underground formations can be determined.
Drill bit vibrations can be resolved into "axial" accelerations, which are accelerations in the direction that the drill is drilling, and "lateral" accelerations, which are accelerations perpendicular to axial accelerations.
There is evidence that lateral waves, which are caused by lateral acceleration, generated below the neutral point (the point at which tension is zero) of the drillstring do not propagate to the surface along the drillstring. Accordingly, the pilot sensor will not detect lateral waves generated by the drill bit.
If, however, lateral waves (generated by the drill bit are of sufficient magnitude, it is likely that the bottom hole assembly ("BHA") will contact the borehole wall at random times and at random locations, in an event called a "wall slap". These contacts will generate strong seismic events that will be detected by the surface sensors and the pilot sensor.
The pilot sensor and surface sensor measurements would not indicate that excessive lateral motion (and hence a possible wall slap) is associated with the event. Such an event would be difficult (and probably impossible) to analyze.